A hydrophone generally involves the following elements: a hydrophone sensing element for generating a signal when subject to pressure changes connected to means for acquiring or transmitting the signal, including a support structure for holding these and optionally shielding for protection of the hydrophone sensing element.
FIG. 1 is a schematic illustration of a hydrophone sensing element, here a fibre-optic air-backed mandrel hydrophone 1. The support structure is here provided by mandrel 2 around which an optic fibre 3 is coiled. The sensitive part of the hydrophone sensing element is here the surface part 4 which is typically cylindrical. The mandrel is mounted on an inner tube 6 to form an air or gas filled cavity in between to make the sensitive element air-backed. Fibre ends 5 provide the means for transmitting the generated signal.
Typical hydrophones are designed to operate in liquid, where one wants the water pressure to induce a large compression of the hydrophone sensing element to provide high sensitivity. In practical use, such hydrophones are shielded or encapsulated for protecting the sensing elements during handling, but care is taken to maintain their sensitivity and dynamic range.
For permanent ocean bottom seismic systems (OBS), hydrophones are typically trenched or buried under the seabed for protection and optimized mechanical coupling to the formations. This imposes a number of challenges. The upper parts of the seabed wherein hydrophones are buried consist of sediments. In the present description, sediment is as a mixture of solid particles and liquid, in the present context typically a mix of sand, silt, or clay and water. Firstly, sediments have, as opposed to water, a finite shear (elastic) stiffness. When the hydrophone is buried in materials with significant shear stiffness, pressure changes to be measured will in addition to cause strain in the sensing element, also cause elastic stress in the surrounding sediments close to the sensing element. These elastic stresses will restrict deflection of the surrounding sediments, and thus potentially reduce the resulting strain in the sensing element, i.e. cause loss in the acoustic pressure energy transfer. The prior art hydrophones are therefore disadvantageous for sensing pressure changes in materials of uncertain or varying shear stiffness. Since the properties of sediments may vary over time as well as with location, the pressure transfer to buried hydrophones, and thus the measured signals, becomes incomparable. Secondly, the hydrophone is subject to a rough handling during burying leading to the need for a stronger protection than required for hydrophones used in water.
U.S. Pat. No. 6,584,038 discloses a hydrophone housing for a solid environment for seismic monitoring of hydrocarbon or gas reservoirs. Here, a hydrophone is immersed in a closed flexible-walled housing filled with liquid and closed by a seal having a duct for a cable connecting the hydrophone to a signal acquisition means. The housing is arranged to be tightly coupled with a solid medium substantially over the total surface thereof, preferably by means of a hardenable material such as cement. There is no mentioning of control of pressure transfer efficiency and hydrophone sensitivity in the patent.
Hence, an improved hydrophone housing with a controlled and stable pressure transfer when operating in sediments, giving a maximum hydrophone sensitivity would be advantageous.